Conclusion Part 1-3

Reverse
voltage diode example shows importance for observing always all
voltages and currents. Do it at least once ! I never thought such a
28VDC winding has to block a large -120V. The theory behind that fact
is simple, but when not thinking about it, problem still exists.
Discussing and review a design with other colleagues, hearing their
proposals is recommended.

My thoughts were: with -100V of the Schottky, there is enough distance
to the absolute maximum rating. My thinking was wrong, because of not
fully understanding this part of the circuit. When not 100% thinking
about the theory => Testing is a must. Without doing this simple
diode measurement such a weak design-issue could go into field.

This diode issue is a typical design-error, nobody would ever find it
on a production or end test. This diode circuit would run in
the field without problems. But the day will come and there is one unit
with a diode less than -120V combined with an temperature or line
voltage extreme, the diode start reverse conducting and run in a
thermal runaway - damage.

When following the four methods, such an design-error can not happen.

(taken from Part 2):

"Best developments methods consider these important issues:

build-up and handmade-evaluated under no missing test equipment and lab conditions.

explanation for each single part: "why using exactly this part and why not another".

explanation Absolute-Maximum-Ratings for each single parameter, how many % have been reached under all conditions.

Point 3.) force the designer to measure the diode voltage and current waveform - designer would find error fast.

Point 4.) if the
designer
forget to measure or don`t understand the measured diode waveforms, the
extended test could brute force this Unknown Error. For e.g. when
testing with a rising-up line voltage, may be at 300VAC rms - BÄNG
and SMOKE -. Extended test force the designer to explain why the diode
has damaged in this test. Designer wants to know why the diode
damages, find it and explain. Also observe full circuit with a
thermal camera, may be the diode is still cold at 290VAC rms, run fast
hot under e.g. 295VAC rms - this is a stop signal for the test to find
the reason first.

For example if the AC
line varistor blows first at 280VAC rms - don´t stop the extended
test! Remove the AC line varistor, renew the melting fuse and
continue testing, increase line voltage further until next Unknown
Error occurs, in our case the blowing diode.

Designer must explain the Unknown Errors - for the line
varistor, easy to explain, varistor does the wanted overvoltage
protection job. For the unknown diode error, after finding the reason
(sometimes hard), designer make a big AHA ! In my experience,
designers will change their design fast as possible within next days.

Prevents from producing e.g. 10000 weak units,
prevents from producing future 5*10000 weak units, because
designers often copy their old design together with the unknown errors
into the new design.

Point 1.) helps the designer to do fast and reliable measurements.

Point 2.) will document all happened issues.

The circuit topology and
concept is the most important key to the design success - but topology
was not the topic of this application note - Purpose was showing
measurements and development methods.